An anion is a substance with a negative charge, and exists in an electrolyte solution or an ionic substance (so-called salt). There are various types of anions such as inorganic anions and organic anions. Among the various types of anions, a polymerizable unsaturated carboxylate anion that is an organic anion is polymerizable in an electrolyte solution such as an aqueous solution, and is often polymerized/cured in an aqueous solution in which it is present as a salt with a metal cation, an organic cation, or the like. A polymerized/cured product produced therefrom is mainly used in aqueous applications. Examples of such polymerizable unsaturated carboxylate anion include (meth)acrylate anions, malate anions, and itaconate anions. In particular, (meth)acrylate anions have high polymerizability, and therefore are industrially important.
The term “salt” refers to an ionic substance which contains an anion and a counter cation, and is electrically neutral (electrically neutralized) as a whole substance. Salts are broadly categorized into two groups: inorganic salts consisting of inorganic substances only and organic salts including an organic substance. Industrially important among organic salts are metal salts of polymerizable unsaturated carboxylic acids, which contain a polymerizable unsaturated carboxylate anion and a metal cation. The metal salts of polymerizable unsaturated carboxylic acids show reactivity attributed to their unsaturated bond and metal salt structure in polymerization/curing reactions and the like, and also give the resulting polymerized/cured product the characteristics derived from the carboxylate anion-metal cation ionic bond and the characteristics derived from the metal itself.
Conventionally used metal salts of polymerizable unsaturated carboxylic acids include metal salts of polymerizable unsaturated carboxylic acids such as (meth)acrylic acid, maleic acid, and itaconic acid. In particular, (meth)acrylate metal salts are industrially important and widely used: the sodium salts and the potassium salts are used as raw materials of aqueous dispersants, detergents, and water-absorbing resins; and the zinc salts are used in wide applications such as rubbers (e.g. tires and golf balls) and as a crosslinking agent for a barrier layer of a gas barrier film. Moreover, (meth)acrylate salts of metals including magnesium, calcium, copper, and aluminum are available as industrial products or reagents. These metal salts of polymerizable unsaturated carboxylic acids enable a metal to be introduced to a resulting polymerized/cured product through the carboxylate anion-metal cation ionic bond (hereinafter, also simply referred to as an ionic bond). Thereby, the resulting product shows various properties such as dispersibility, water absorbability, high hardness, high elasticity, and gas-barrier property, which are the properties derived from one of or both of the ionic bond and the metal.
On the other hand, concerning polymerizable unsaturated carboxylic acid-based compounds for example, Non Patent Literatures 1 and 2 disclose compounds in which an allyloxymethyl group is introduced to α-position of an acrylic ester such as ethyl α-allyloxy methyl acrylate or methyl α-allyloxy methyl acrylate. These compounds are polymerizable compounds which contain a double bond activated by the conjugation with an adjacent carbonyl group, like in the case of (meth)acrylic esters. The Non Patent Literatures disclose that these compounds are cyclopolymerized by a radical addition polymerization mechanism, and soluble polymers are generated.